Make mojo::Callback safe to copy across threads.

mojo/public/cpp/bindings/message.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef MOJO_PUBLIC_CPP_BINDINGS_MESSAGE_H_
 #define MOJO_PUBLIC_CPP_BINDINGS_MESSAGE_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
@@ skipping 119 matching lines @@
 // A MessageReceiver that is also able to provide status about the state
 // of the underlying MessagePipe to which it will be forwarding messages
 // received via the |Accept()| call.
 class MessageReceiverWithStatus : public MessageReceiver {
  public:
   ~MessageReceiverWithStatus() override {}
 
   // Returns |true| if this MessageReceiver is currently bound to a MessagePipe,
   // the pipe has not been closed, and the pipe has not encountered an error.
   virtual bool IsValid() = 0;
+
+  // DCHECKs if this MessageReceiver is currently bound to a MessagePipe, the
+  // pipe has not been closed, and the pipe has not encountered an error.
+  // This function may be called on any thread.
+  virtual void DCheckInvalid(const std::string& message) = 0;
 };
 
 // An alternative to MessageReceiverWithResponder for cases in which it
 // is necessary for the implementor of this interface to know about the status
 // of the MessagePipe which will carry the responses.
 class MessageReceiverWithResponderStatus : public MessageReceiver {
  public:
   ~MessageReceiverWithResponderStatus() override {}
 
   // A variant on Accept that registers a MessageReceiverWithStatus (known as
@@ skipping 16 matching lines @@
 // the caller should wait on the handle to become readable. Returns
 // MOJO_RESULT_OK if the message was read successfully and should be
 // dispatched, otherwise returns an error code if something went wrong.
 //
 // NOTE: The message hasn't been validated and may be malformed!
 MojoResult ReadMessage(MessagePipeHandle handle, Message* message);
 
 }  // namespace mojo
 
 #endif  // MOJO_PUBLIC_CPP_BINDINGS_MESSAGE_H_